1. Field of the Invention
This invention relates to a radiation image readout method for use in a radiation image recording and reproducing system, and a radiation image recording and read-out apparatus for carrying out the method. This invention particularly relates to a method of adjusting read-out conditions and/or image processing conditions for use in a radiation image recording and reproducing system, and a radiation image recording and read-out apparatus for carrying out the method, in which a stimulable phosphor sheet is circulated and reused for recording radiation images.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultra-violet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet comprising the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays which cause it to emit light in proportion to the radiation energy stored. The light emitted from the stimulable phosphor sheet when the sheet is exposed to the stimulating rays is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy. The finally obtained visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT). In this radiation image recording and reproducing system, the stimulable phosphor sheet is used to temporarily store the radiation image in order to reproduce the final visible image therefrom in a final recording medium. For economical reasons, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
Further, in a mobile X-ray diagnostic station such as a traveling X-ray diagnostic station in the form of a vehicle like a bus which is provided with a radiation image recording and read-out apparatus for use in the aforesaid radiation image recording and reproducing system and moves from place to place to record radiation images for mass medical examinations, it is disadvantageous to load a mobile X-ray diagnostic station with a number of stimulable phosphor sheets, and the amount of the stimulable phosphor sheets which can be loaded on the mobile X-ray diagnostic station is limited. Therefore, it is desired to load the mobile X-ray diagnostic station with stimulable phosphor sheets which can be used repeatedly, once store the radiation images of the objects in the stimulable phosphor sheets, transfer the electric image signals read out from the stimulable phosphor sheets into a recording medium having a large storage capacity, such as a magnetic tape, and circulate and reuse the stimulable phosphor sheets for further image recording and read-out operations, thereby to obtain the radiation image signals of many objects. Further, when image recording is conducted continuously by circulating and reusing the stimulable phosphor sheets, it becomes possible to increase the image recording speed in mass medical examination. This is very advantageous in practical use.
In order to reuse stimulable phosphor sheets as described above, the radiation energy remaining in the stimulable phosphor sheet after it is scanned with stimulating rays to read out the radiation image stored therein should be eliminated or erased by the method as described, for example, in Japanese Unexamined Patent Publication No. 56(1981)-12599 or U.S. Pat. No. 4,400,619. The stimulable phosphor sheet should then be used again for radiation image recording.
Accordingly, it is desired that there be combined into a single apparatus: an image recording section for exposing each circulatable and reusable sheet composed of a stimulable phosphor to a radiation passing through an object, an image read-out section for reading out the radiation image stored in the stimulable phosphor sheet, and an erasing section for erasing the radiation energy remaining in the stimulable phosphor sheet after the readout step for the purpose of again recording another radiation image therein. This is because such an arrangement would make it possible to easily load the apparatus on the mobile X-ray diagnostic station to conduct medical examinations at different locations. Such an apparatus could also be easily installed in a hospital r the like. This is very advantageous in practical use.
The radiation image recording and read-out apparatus using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the stimulable phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor to the radiation by reading out the emitted light with an appropriate read-out gain and converting it to an electric signal to reproduce a visible image on a recording medium or a display device. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
Also, in the radiation image recording and read-out apparatus using a stimulable phosphor sheet, compensation for deviation of the level of the radiation energy stored in the stimulable phosphor sheet from a desired level can easily be carried out by adjusting the read-out gain to an appropriate value when photoelectrically reading out the light emitted from the stimulable phosphor sheet upon stimulation thereof. Therefore, the quality of the reproduced radiation image is not adversely affected by a fluctuation in radiation dose due to fluctuating tube voltage or MAS value of the radiation source, a variation in the sensitivity of the stimulable phosphor sheet or the photodetector, a change in radiation dose according to the condition of the object, or a fluctuation in the radiation transmittance according to the object, and the like. Also, it is possible to obtain a desirable radiation image even when the radiation dose to the object is low.
However, in order to eliminate various influences caused by the fluctuation of radiographic exposure conditions and/or to obtain a radiation image having a high image quality or a high diagnostic efficiency and accuracy, it is necessary to investigate such image input conditions of the radiation image stored in the stimulable phosphor sheet as, for example, the level of radiation dose used for image recording, or the image input pattern which is determined by the portion of the body (e.g. the chest or the abdomen) or the image recording method used, such as plain image recording or contrasted image recording, before conducting final read-out for obtaining radiation image information for reproducing the radiation image to a visible image, and then to adjust the read-out conditions such as the read-out gain to appropriate values or to process the electric signal appropriately on the basis of the detected radiation image information. It is also necessary to determine the scale factor to optimize the resolution according to the contrast of the image input pattern.
Grasping of the radiation image information prior to the final read-out and adjustment of the read-out conditions on the basis of the grasped information may be conducted by use of the method as disclosed in U.S. Pat. No. 4,284,889, which is based on the finding that the amount of light instantaneously emitted by the stimulable phosphor sheet upon exposure thereof to a radiation is proportional to the amount of the radiation energy stored in the stimulable phosphor sheet. In this method, the amount of light instantaneously emitted by the whole surface of the stimulable phosphor sheet is detected by use of many photodetectors, and the read-out conditions are adjusted on the basis of the maximum and the minimum of the instantaneous light emission amount.
However, in the aforesaid method, since many photodetectors are used for detecting the amount of light instantaneously emitted by the whole surface of the stimulable phosphor sheet, the configuration of the apparatus for carrying out the method becomes complicated. Further, it is not always possible to correct the sensitivity of each photodetector and to control photodetectors so that all the photodetectors exhibit the same sensitivity.
In the aforesaid radiation image recording and read-out apparatus, in order to obtain an image having an improved image quality, particularly a high diagnostic efficiency and accuracy, it is desirable to approximately grasp in advance the radiation image information stored in the stimulable phosphor sheet, and to photoelectrically detect the light emitted by the stimulable phosphor sheet for obtaining an image signal by use of read-out conditions adjusted on the basis of the grasped image information, or to process the image signal by use of image processing conditions adjusted on the basis of the grasped image information. When the radiation image recording and read-out apparatus is fabricated for conveying in the aforesaid mobile X-ray diagnostic station, the apparatus should be simplified as much as possible. However, the method of approximately grasping in advance the radiation image information by detecting the light instantaneously emitted by the whole surface of the stimulable phosphor sheet by use of many photodetectors is not suitable for this purpose since the apparatus becomes complicated, the cost becomes high and it is not always possible to control photodetectors so that all of the photodetectors exhibit the same sensitivity. Thus a need exists for a method of more simply grasping the image information.